1. Field of the Invention
The present invention relates to a voltage controlled oscillator (VCO) employed in a frequency synthesizer of a low-power system. More particularly, the present invention relates to a voltage controlled oscillator with body bias control which can generate an oscillation signal of a uniform level without being affected by changes in a supply power.
2. Description of the Related Art
Recently the IEEE 802.15.4 (ZigBee) standard has garnered attention and found its application in home network, security and logistics where short-distance and low-speed transmission is required. A telecommunication system applied to such applications characteristically needs to run on one long-lasting battery, and thus at low power.
Meanwhile a frequency synthesizer of an RF transceiver operates during both transmission and reception to generate a local oscillation frequency, thereby consuming power considerably. In this frequency synthesizer, most power is dissipated by a voltage controlled oscillator (VCO) and a divider. Accordingly, studies are under way to reduce current dissipated by two blocks of the voltage controlled oscillator and the divider.
Most of all, recently, studies have been vigorously conducted to reduce power by a lowered supply voltage rather than by a structural approach.
FIG. 1 is a circuit diagram illustrating a conventional voltage controlled oscillator.
The conventional voltage controlled oscillator of FIG. 1 includes a resonant circuit 11 and a differential cross-coupled oscillation circuit 12. The resonant circuit 11 resonates at a supply voltage Vdd of about 1.8V to 3.3V and generates a resonant frequency in response to a tuning voltage VT. The differential cross-coupled oscillation circuit 12 supplies energy to the resonant frequency from the resonant circuit 11 to generate first and second signals Sout1 and Sout2 having a phase difference of 180 degree.
The differential cross-coupled oscillation circuit 12 includes a pair of first and second differential cross-coupled transistors M1 and M2. Here, the first transistor M1 has a drain connected to a first end of the resonant circuit 11 and a gate of the second transistor M2, a gate connected to a second end of the resonant circuit 11 and a drain of the second transistor M2, and a source connected to a ground. The second transistor M2 has a drain connected to a second end of the resonant circuit 11 and the gate of the first transistor M2, a gate connected to the first end of the resonant circuit 11 and the drain of the first transistor M2, and a source connected to the ground.
Also, the first and second transistors M1 and M2 each have a body bias connected to the ground.
Such a voltage controlled oscillator is typically referred to as a differential cross-coupled LC-tuned VCO.
But the conventional voltage controlled oscillator, when used at a low supply voltage of about 0.5V, cannot adopt a stack structure with many transistors. Accordingly, the voltage controlled oscillator is structured such that an inductor is connected to a negative gm cell via load.
In this case, the lower supply voltage considerably narrows a frequency range of an oscillator. Also, in this structure, many capacitor arrays are used to accommodate a desired frequency band, thereby creating parasitic components significantly.
Furthermore, to ensure stable oscillation, the negative gm cell should be designed in a large size. This noticeably changes an oscillation frequency and the oscillator performs sensitively in response to changes in the supply voltage.
Therefore, there is a demand for a voltage controlled oscillator of a new structure which can compensate for changes in the supply voltage and oscillate stably.